This invention relates to a fiber optic optical transducer, and more particularly, the invention relates to such a transducer which produces digital information.
Electrically passive optical transducers or encoders using fiber optic components have become of considerable interest for a wide variety of applications. Numerous applications exist for rotary and linear encoders which are capable of producing digital information. In order, however, for these encoders to find widespread application, it was determined that multiplexing of the encoder signals is required to reduce the number of fibers and to improve reliability.
The first effort in this direction used time division multiplex techniques. In such an arrangement, an optical fiber delivers a short duration optical pulse to the encoder. The encoder uses fiber optic couplers and fiber optic delay lines to time multiplex the binary encoder output signals as a sequence of optical pulses. This approach has several disadvantages which are all primarily associated with the need for the use of fiber optics delay lines. Conventionally, a delay line spool would be used, and this was found to be bulky and expensive to wind. Moreover, the fibers arranged in this configuration are subject to breakage when exposed to temperature extremes.
These and other difficulties with time division multiplexing caused experimentation with and the subsequent use of wavelength division multiplexing techniques.
Fiber optic encoders for creating the encoded wavelength division multiplex signal generally involve the use of a grated index rod lens (GRIN lens) having a glass wedge-grating assembly arranged at an end thereof. Broadband light enters the encoder system through the encoder input fiber and passes through a fiber coupler to the multiplexer/encoder. The multiplexer disperses the broadband spectrum of received light across the channels of a reflective code plate in wavelength bands. Those wavelengths directed to a channel in the logic zero state are, for example, absorbed by the code plate, while those wavelengths directed to a channel in the logic one state are, for example, reflected by the code plate and then retransmitted to the multiplexer input/output fiber by the grating-lens assembly. A coupler then directs the reflected light to a separate encoder output fiber for transmission from the encoder. Dual transmission fibers communicate the reflected light information to a receiver/demultiplexer. The demultiplexing operation is performed by a second grating assembly which, for example, disperses the spectrum on to a photo diode array. In the latter, each diode, for example, might correspond with a given wavelength band. The totality of the array then corresponds with a given code word, and the logical values of the bits forming the code word are determined by whether or not the elements of the array are actuated, i.e., whether or not there is reflected light appearing in the corresponding wavelength bands.
In these prior art fiber optic transducer assemblies, the decision as to whether a given photodiode is to be actuated indicating light in the corresponding wavelength band is generally by comparison of the value of the reflected light with some t value. It can be readily seen that in a system such as there are ample possibilities for power level variations for example, the light source and along the lengths of the fiber optic cables. Consequently, system power losses and corresponding level variations have produced errors in the presence or absence of reflected light in a given band. The errors produced by these threshold comparisons cast considerable doubt on the efficacy of this technique optically encoding sensed information.